The present invention relates generally to optical laser telescopes, and more specifically to a digital position monitor which is used to output an X and Y position of a laser beam on a quadrant or lateral effect detector.
The subject matter of this application is related to the subject matter contained in the following U.S. patent application Ser. No. 0/7,500,614, filed 13 Mar. 1990, entitled "Pointer/Tracker for a Phased Array Laser System" by Mark Baciak, the disclosure of which is specifically incorporated herein by reference.
In the prior art, the use of multiple telescopes in a synthetic array has encountered a number of problems. Dynamic off axis pointing of arrays of multiple telescopes is difficult to align and maintain since such individual telescope must have its phase dynamically adjusted to point its laser output in a steerable direction that coincides with the rest of the telescopes in the array. One prerequisite of this phase adjustment is that the actual direction of all the telescopes must be measured and compared before the adjustment requirements are determined.
The task of measuring the directional outputs of individual laser telescopes is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,639,586 issued to Fender et al; PA1 U.S. Pat. No. 4,667,090 issued to Carreras et al; PA1 U.S. Pat. No. 4,689,758 issued to Carreras; PA1 U.S. Pat. No. 3,670,168 issued to Low; and PA1 U.S. Pat. No. 3,825,845 issued to Angelback.
The above-cited references are examples of prior art. The Fender apparatus performs phase matching between pairs of laser beams using an array containing at least two optical telescopes which become useable as a laser transmitter when combined with an optical phase matching system consisting of: a collecting telescope, a detector array, two fold mirrors, analog-to-digital converter, microprocessor, and two sets of correcting mirrors.
The Carreras reference provides a detailed apparatus and technique to calculate the optical phase difference for two signals from an interference pattern provided by the CCD camera. The Carreras system has been successfully used with the Fender et al system to determine the difference between two beams using one of a number of phase estimating algorithms.
Once an accurate estimate of the phase difference is determined, the estimated error is used to adjust the phase by the correcting mirrors which adjust the optical path lengths of the two outgoing beams. This approach is also used in the system of the Carreras et al reference. While the Carreras et al system provides improved tilt control, it offers limited flexibility, particularly when all telescopes are forced to dynamically track a reference. This is because the original tracker designed for the phased array system implemented a concept of the common physical element. The theory was that low frequency drifts in electronics, optics, and mechanical components caused distortion of the point spread function in the far field pattern. It was felt that if the optics and electronics were common to all three telescopes that the long term distortion effects could be controlled. That was true to a point, but the tracker offered limited capability. Off axis pointing presented a new problem that was beyond the capability of the tracker. Additionally, the tracker required complicated electronic circuitry that was difficult to align and maintain. It also caused ambiguities and instabilities in the tilt control loops.
Another system which may be used for rapid optical phasing of optical devices using white light interferometry is disclosed in the patent application of Mark Baciak entitled "Method and Apparatus for Rapid Optical Phasing", U.S. patent application Ser. No. 857,621, the disclosure of which is incorporated herein by reference. The system of the Baciak `621` reference includes a white light source, a beam splitter, a scanning mirror and a stationary reflective surface. The scanning mirror is moved in piston while reflected light is observed by an electro-optic sensor at a receiver. A nulled pattern has been previously established at the receiver. An electrical signal from the electro-optic sensor is mounted at an amplification and recording means to identify variations in reflected light intensity that are caused by white light fringes. White light fringes only occur at zero optical difference, i.e. when the scanning mirror and stationary reflective surface are in phase.
The Baciak `621` reference is important because it discloses a method of phasing a plurality of optical devices. The method comprises transmitting light through a beam splitter to a reference mirror and an optical device to be phased. Light reflected from the reference mirror and the optical device are recombined at the beam splitter and directed to a receiver. Light at the receiver is converted by an electro-optical sensor to an electrical signal which is transmitted to a detection and amplification means. The optical device is then moved along the path of light directed to it (i.e., in piston) from the beam splitter until a variation in light intensity at the receiver indicates that there is zero optical difference between the reference mirror and the optical device being tested. The first optical device tested can then be considered a reference and this process repeated with other optical devices to be phased.
Another exemplary phase matching system is disclosed in U.S. Pat. No. 4,600,308 by T. Waite entitled "Phase-Matching Arrayed Telescopes with a Corner-Cube-Bridge Metering Rod" the disclosure of which is incorporated herein by reference. The Waite system resembles that of Janet Fender in that is uses an optical trombone to optically adjust the phase of adjacent telescopes through changes in optical path lengths.
All of the references cited above demonstrate the interest in a relatively emerging technology of phased arrays of optical telescopes. The current trend of developments resemble a similar development in phased array radar systems experienced a decade ago. The present invention is believed to contribute to the current developments by providing a digital position monitor which outputs a signal which represents the position of a laser beam, and the direction in which it is pointed. As mentioned above, such dynamic digital measurements are a prerequisite to making phase adjustments to steer a synthetic array of multiple telescopes.